Pie charts are used to display the contribution of fractional parts to a whole. Pie charts are most commonly
used to make comparisons between groups. They have no axes. When a numeric field is used, the chart can calculate
the percentage of each value to the total.

Pie charts are popular in reports because of their visual impact. However, pie charts are a very
simplified chart type that may not best represent your data. Consider using a pie chart only after the data has
been aggregated to seven data groups or less.

Pie charts display each data group as a separate slice on the chart. You must add at least one data
field and one series field to the pie chart. If more than one data field is added to a pie chart, the pie chart
will display both data fields in the same chart.

If you are defining your own colors on a pie chart using a custom palette, be sure that you have
enough colors in your palette to display each data point with its own unique color.

A pie chart requires at least two values in order to make a valid comparison between proportions.
If your pie chart contains only one color, verify that you have added a series field to group by. When the pie chart
does not contain series, it aggregates the values from your data field into one value for display.

Pie charts represent proportions of the total. As a result, it is common to format pie chart
labels as percentages. In order to be consistent with other chart types, the chart does not display percentage labels by default.
To show labels with percentages follow these steps:

Select the series by clicking anywhere on the pie.

Set DataPointLabelStyle.Visible property of the series from the Properties window to true.

Set DataPointLabel property of the series to an expression adjusted to your specific needs similar to the following one: =Sum(Fields.LineTotal)/Exec('graph1', Sum(Fields.LineTotal)) where 'graph1' is the name of the Graph item.

Select the series labels and from the 'Graph Tools | Format' contextual tab select 'Percent' for the label format.

Adjust the precision according to your needs.

Position the labels through the DataPointLabelAlignment property of the series.

By default, the layout engine will try to arrange the data labels so they do not overlap. When the bounds of two or more labels overlap,
the engine will move them and their adjacent labels vertically, trying to find them a proper non-overlapping positions. During this rearrangement,
a label can be moved aside from its original location, which may produce a hard to read chart. Additionally, if the plot area doesn't provide
enough space and there are a lot of data points, the labels will overlap. There are several ways to prevent labels from overlapping:

Decrease the font size of the data point labels.

Increase the width and height of your chart to allow more room for the labels.

Change pie labels position through the DataPointLabelAlignment property of the series.

Set the DataPointLabelOffset to a greater value
so the labels will be arranged around a circle with a bigger radius.

The algorithm that moves the data point labels, preventing them from overlapping, is activated only when the labels have their
DataPointLabelAngle set to a multiple of 2*π radians in degrees (i.e. 0, 360, etc.).

Reference

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